In conventional motorcycles, a throttle body is coupled to an intake port of an engine, and a butterfly throttle valve disposed in an air-intake passage of the throttle body is controlled to be opened and closed, thereby controlling an amount of air taken in from outside and supplied to the engine. The throttle valve is opened and closed in association with a rider's hand operation of a throttle grip of the motorcycle. If a change amount in an opening degree of the throttle valve in response to the rider's throttle grip operation is large, then the amount of air varies significantly, causing the rider to feel discomfort during travel of the motorcycle. If the rider quickly closes the throttle grip to close the throttle valve, then the amount of air becomes insufficient for stable combustion. As a result, gas exhausting efficiency decreases.
As a solution to this, there has been disclosed a throttle valve controller configured to cause a motor to open and close the throttle valve in addition to the rider's hand operation to enable phase angle control of the throttle valve (see e.g., Japanese Laid-Open Patent Application Publication No. Hei. 2-5716 or Publication of Japanese Examined Patent Application No. Hei. 3-64694). The throttle valve controller is configured to calculate an optimal target opening degree of the throttle valve depending on an operating state of a vehicle and to cause a motor to electronically control the throttle valve to minimize a deviation between a valve opening degree in response to the rider's hand-operation and the target opening degree.
However, if the motor does not correctly operate and unexpectedly stops in a state where the throttle valve is driven to be opened by the motor, then the throttle valve will be left open by an excess phase angle due to the stopping of the motor. Under this condition, if the rider returns the throttle grip to a position corresponding to a fully closed position of the throttle valve, the throttle valve may remain opened by the excess phase angle, which may not correspond to the suitable throttle position for an idling engine speed in a normal state. As a result, gas exhausting efficiency and fuel consumption efficiency decrease.